I've never seen an ad for a book in a magazine or heard one on the radio. I guess I'm reading the wrong magazines and listening to the wrong stations. The local newspaper doesn't pick up the ads, and I'm not buying the NYT just for the book reveiws.
---I'm not talking about advertisements. I'm talking about editorial content (i.e., a magazine article about an author, a review of a book; a radio interview of an author, a discussion of a book on TV). All of these things are very common in major media. The way to get them is to get the people who make decisions on what kinds of stories/articles/features to print or put on the air. Those people are called editors. Think Oprah's Book Club. Think "Fresh Air", the radio program on NPR. The Books section in your local newspaper or magazine, or even the New York Times Book Review.
Again, it's not advertising, which you pay for. This is editorial content, and it's free. The trick is getting the decisionmakers in charge of the content to notice you.